1. Technical Field of the Invention
The present invention relates to a method and a system for washing incineration ash generated when urban garbage and the like is burned and dust contained in combustion gas extracted from a kiln exhaust gas passage, which runs from an inlet end of a cement kiln to a bottom cyclone.
2. Description of the Related Art
In view of risk on the exhaustion of final disposal sites, incineration ash generated at the burning of urban garbage and the like is recycled for a cement raw material in recent years. Fly ash that is included in the urban garbage incineration ash and is transported together with gas and recovered by a dust collector contains chlorine component from 10 to 20 percent, which necessitates removal of chlorine component before recycled for a cement raw material. Then, the incineration fly ash is utilized for a cement raw material after soluble chloride compound included in the ash is removed through washing in a washing and desalting facility such as a belt filter.
Meanwhile, it is noticed that chlorine, sulfur, alkali and the like cause troubles such as preheater clogging in cement manufacturing facilities, and especially chlorine has the most harmful effect, so that a chlorine bypass system is used in order to remove chlorine by extracting a part of combustion gas from a kiln exhaust gas passage, which runs from an inlet end of a cement kiln to a bottom cyclone.
In this chlorine bypass system, for example, as described in the patent document 1, more chlorine is distributed on fine powder side of the dust, which is generated by cooling the extracted exhaust gas, so that the dust is separated into coarse powder and fine powder by a classifier, the coarse powder is returned to a cement kiln system, and the fine powder (chlorine bypass dust) containing separated potassium chloride and the like are recovered, and are added to a cement grinding mill system.
However, in recent years, recycling of waste including the incineration ash through conversion to cement raw material or fuel has been promoted and the quantity of the treated waste increases, which increases the quantity of volatile component such as chlorine brought to cement kilns. As a result, the quantity of the generated chlorine bypass dust increases also. Therefore, all of the chlorine bypass dust cannot be utilized in cement grinding process, so that the chlorine bypass dust also has been treated by washing.
In addition, the quantity of heavy metals brought to cement kilns increases as the quantity of the treated waste at cement manufacturing facilities increases, which may cause heavy metals to exceed allowable concentrations in cement. Therefore, for example, in a treatment for converting into cement raw material described in the patent document 2, desalting chlorine bypass dust and the like, which was washed in the past; adding water to waste including chlorine to allow chlorine in the waste to elute; filtrating the slurry to utilize obtained desalted cake as a cement raw material; removing heavy metals such as copper and lead by purifying the filtrate to promote effective utilization of chlorine bypass dust without causing environmental pollution.
Meanwhile, into the cement manufacturing processes are brought selenium (Se) and thallium (Ti) in addition to copper, lead and the like described above. For example, pulverized coal supplied to cement kilns and calciners contains approximately 1 ppm of thallium, and waste tires supplied thereto contains approximately 8 ppm of thallium. Since the thallium has a low boiling point and volatilizes between a kiln and a preheater of a cement burning device and most of the thallium concentrates at the preheater, as a result, the thallium is included, for instance, in a wastewater after chlorine bypass dust is treated.
As described above, conventionally, to recycle urban garbage incineration ash and the like as cement raw material, it might be necessary to remove chlorine component from fly ash and chlorine bypass dust and to remove heavy metals such as thallium, lead, selenium and the like from a filtrate obtained by washing the chlorine bypass dust, so that it is necessary to install a plurality of treatment facilities and to place workers to each treatment facility, which causes drastic rises of facility cost and running cost.
In this regard, in the patent document 3 is described a method for suppressing facility cost and running cost low when the urban garbage incineration ash and the like are recycled as a cement raw material by simultaneously performing washing incineration ash and chlorine bypass dust, and removing one or more selected from thallium, lead and selenium eluting in a filtrate obtained after the washing by adding sulfurizing agent and/or reducing agent to the filtrate.
Patent document 1: World Patent Publication WO97/21638 pamphlet
Patent document 2: Japanese Patent Publication 2000-281398 gazette
Patent document 3: Japanese Patent Publication 2007-268398 gazette
However, the quantity of the generated chlorine bypass dust has recently been increasing with further increase of the quantity of the treated waste including incineration ash described above. Thus, in case that incineration ash and chlorine bypass dust are simultaneously washed with the incineration ash treating method described in the patent document 3, medical agents for removing selenium and thallium, which are included in the chlorine bypass dust but are not included in the incineration ash, disperse allover the filtrate after the washing, so that a plenty of medical agent is consumed, resulting in drastic rise of medical cost. In addition, when mixing the filtrate after washing the incineration ash, which has high calcium concentration, and the filtrate after washing the chlorine bypass dust, which has high SO4 concentration, with each other, calcium sulfate (CaSO4) is generated, which prevents stable operation of a filtering device and a downstream wastewater treatment process due to scale adhering thereto. On the other hand, when independent washing facilities for separately washing the incineration ash and the chlorine bypass dust are installed, this causes facility cost and running cost to double.